When opening a world, redstone takes a couple of ticks to update.

When opening a world, it takes redstone a couple of ticks to update to what it should be. For example, if a redstone torch is supposed to be burnt out, and it is burnt out when you exit the world, when you join the world again, the torch will be lit for a couple of ticks before it becomes burnt out again. This ruins most creations built around moving items as the items may move as you rejoin the world.